Conventionally, in a communication system which employs mobile stations such as portable terminals, a plurality of frequencies are defined as frequencies for downlink signals transmitted from a base station to a mobile station. Then, one or a plurality of frequencies is selected from among a plurality of these frequencies, and a downlink signal is transmitted using the selected frequency (frequencies).
FIG. 1 is a diagram schematically showing a frequency domain for describing a band search which is a conventional frequency domain search.
For example, in 3GPP (3rd Generation Partnership Project) which is a standard of W-CDMA (Wideband Code Division Multiple Access), 276 frequencies called raster are set at intervals of 200 kHz in a frequency domain from 2110 MHz to 2170 MHz except for 2.5 MHz at both ends, as shown in FIG. 1. Then, an effective frequency is selected from the set frequencies, and a downlink signal is transmitted using a transmission band centered at the selected effective frequency. In this regard, the raster is defined as a minimum unit for placing a center frequency within a transmission band of a system.
A mobile station, in turn, detects an effective frequency from among candidate frequencies upon power-on or upon detection of being out-of-range, and further establishes synchronization with a base station. Processing for detecting this effective frequency is called “band search processing.” The detection of an effective frequency may involve using a known signal called a synchronization signal. As a method for speeding up this band search processing, a method has been proposed for grouping a plurality of adjacent frequencies into a block (for example, see JP-2003-244083-A).
Also, in 3GPP Release 7, consideration has been given to enabling a plurality of transmission bandwidths (1.25, 2.5, 5, 10, 15, 20 MHz) to be set from a narrow band to a wide band within a frequency band owned by an operator (for example, see 3GPP TR 25.814.V1.1.1 (2006-2) Physical Layer Aspects for Evolved UTRA (Release 7) Section 7.1.1).
Further, a proposal has been made with respect to such a system which can set a plurality of bandwidths to match center frequencies of a plurality of bandwidths, where the center frequencies are set to be integer multiples of a raster, and placing a synchronization signal (SCH) in a central band (for example, see 3GPP R1-060311 SCH Structure and Cell Search Method for E-UTRA Downlink).
On the other hand, in recent years, OFDM (Orthogonal Frequency Division Multiplexing)/OFDMA (Orthogonal Frequency Division Multiplexing Access), which excels in multipath resistance, tend to be employed in mobile communications, including 3GPP Release 7, 3GPP Long Term Evolution (LTE), and WiMAX as well. In this event, since parameters such as a sub-carrier interval are set taking into account to fading resistance, the sub-carrier interval can fail to be an integer multiple of a raster, resulting in difficulties in simplification of band search processing and synchronization processing.
However, the method described above implies a problem in which a long time is required for band searching processing to detect an effective frequency because the method sequentially searches a number of set candidate frequencies for the presence or absence of effective waves.
Also, a large amount of processing is required for sequentially searching a number of frequencies for the presence or absence of effective waves, and moreover, when OFDM is used as a transmission scheme, intermediate results and the like cannot be referenced to one another in the processing for each candidate frequency unless the sub-carrier interval is an integer multiple of the raster, giving rise to a problem of a failure to reduce the processing amount and a resulting increase in power consumption required for band search processing.